Rubber compositions can be reinforced to show higher modulus (complex elastic modulus) by incorporating microfibrillated plant fibers such as cellulose fibers as filler into the rubber compositions. However, since microfibrillated plant fibers are highly self-aggregative and poorly compatible with rubber components, they are less likely to be dispersed in rubber during kneading. For this reason, the incorporation of microfibrillated plant fibers can deteriorate tensile properties or fuel economy in some cases. Therefore, there is a need for methods for improving the dispersibility of microfibrillated plant fibers.
Another problem is that when a water dispersion of cellulose fibers is dried, the cellulose fibers are aggregated, and thus such cellulose fibers cannot be dispersed at the nano level when they are directly mixed with rubber. For this reason, it is difficult to improve tensile strength and to reduce rolling resistance. Methods for solving this problem have been proposed (for example, Patent Literature 1) in which cellulose fibers are dispersed in rubber by dispersing the cellulose fibers in water beforehand and adding rubber latex to the dispersion, followed by stirring and then drying.
Other attempts have been made to improve the dispersibility of cellulose fibers in rubber by using a compatibilizer, by modifying cellulose fibers, or by using a fibrillating resin.
For example, Patent Literature 2 discloses a method for improving the compatibility of microfibrillated cellulose with rubber by chemically modifying the microfibrillated cellulose.
Patent Literature 3 discloses a method for improving the dispersibility of cellulose fibers in a rubber component. According to this method, cellulose fibers are modified by introducing a vinyl group therein, via which a cross linkage can be formed between the cellulose fibers and the rubber component to increase the affinity therebetween.